This invention relates to a method and apparatus for making continuous metal strips, particularly metal strips with an amorphous molecular structure, by depositing molten metal onto the moving surface of a chill body by forcing the metal through a slotted nozzle located in close proximity to the surface of the chill body.
For purposes of the present invention, a strip is a slender body whose transverse dimensions are much less than its length, including wire, ribbons and sheets, of regular or irregular cross section.
There has long been recognized the need for processes which would permit manufacture of finished or semifinished products such as wire, ribbon or sheet directly from the molten metal. Hubert et al. provided a review of such processes and classified then available technology into the "melt spin process" and the "melt drag process", [Zeitschrift fuer Metallkunde 64, 835-843 (1973)].
In the melt spin process, a jet of molten metal is cooled, either in free flight or by jetting it against a chill block, to obtain continuous filament. Both of these embodiments employ a pressurized orifice. There is also a melt spin process operating without an orifice, wherein molten metal is supplied to a jet-forming device, such as a grooved spinning disc, to be expelled therefrom. Hubert et al. stated that the key to success in the melt spin process is to stabilize the liquid jet until it solidifies. Jets of molten metal are inherently unstable since they have a strong tendency for droplet formation on account of the low viscosity and high surface tension of molten metal. Basic problems of jet stability have been discussed by Butler et al., in Fiber Science and Technology 5, 243-262 (1972).
In the melt drag process (see U.S. Pat. Nos. 3,522,836 and 3,605,863, both to King molten metal is made to form a meniscus held on by surface tension, at the outlet of a nozzle. From this meniscus molten metal is then dragged onto a rotating continuous cooled drum or belt. This method avoids the difficulties of jet instability inherent in the melt spin process. Unfortunately, however, the speed of the moving chill surface in the melt drag process is severely restricted due to restriction in melt flow at the meniscus, or else only discontinuous filament is obtained. Also, it is believed that the melt drag process is not readily adaptable to provide sufficiently high rates of cooling of the molten metal to permit production of amorphous metal strips. Such strips require rapid quenching of certain molten alloys at a cooling rate of at least 10.sup.4 .degree. C. per second, more usually 10.sup.6 .degree. C. per second.
Continuous amorphous metal strips of narrow widths and thicknesses have heretofore been made by the melt spin process involving rapid quenching of a jet of molten metal directed against a moving chill surface, such as the inside or outside of a rotating roll, or a moving belt. The molten alloy jet which is to be quenched is stable due to its high velocity for a relatively short distance, say 3 to about 6 millimeters. When it hits the rapidly moving chill substrate (velocity typically between about 1300 and about 2000 meters per minute) it wets it and forms a puddle. That puddle is essentially stationary in space as the moving substrate draws it out into a strip, which is traveling at the same speed as the substrate. In actual application, it has been found that, using a single jet, the maximum width of strip so obtained from a jet of substantially circular cross section is limited to about 5 to 6 millimeters. Attempts to provide wider strips by impinging a sheet-like jet against a moving chill surface met with little success, principally because the wide jet initially does not form a smooth line puddle as required to obtain a uniform, wide product, and consequently produces kinky, nonuniformly quenched strip.
It is also possible to impinge a plurality of parallel, uniform jets properly spaced onto a moving substrate to form a relatively wide strip. This approach, however, has inherent difficulties inasmuch as it requires a close matching of the jet velocities and spacings with the substrate speed. The principal difficulty is that either the jets do not join together to form a puddle, or the jets run together to form a ridge, so that, from a practical standpoint, it is difficult to obtain strips with uniform cross sections. Moreover, since the molten metal puddle deposited by the jets onto the chill substrate tends to assume the equilibrium shape of a droplet, thick at the center and thin at the edges, it is very difficult, if not impossible, to maintain a puddle of sufficiently uniform thickness for "drawing out" strips having even approximately uniform cross section wider than about 7.5 mm.
In any event, it has not been possible to obtain wide metal strips, say wider than about 6 millimeters, by single or multiple jet casting procedures having isotropic strengths, that is to say having identical tensile strengths and elongation measured in the transverse as well as in the longitudinal direction, or in any direction therebetween, even though metal strips with amorphous structures should be isotropic at least with respect to their tensile properties, and those with cast polycrystalline structures should be approximately isotropic. Anisotropic tensile properties of wide strips of amorphous metal obtained by multiple jet casting procedure are believed to be caused by inherent imperfections in the strips obtained by that procedure. Significantly, however, strips made by jet casting procedures, regardless of width, lack uniformity of thickness, measured transversely, and they are prone to show significant variations in width along their length. They lack such uniformity of thickness because they are drawn out from a puddle of liquid metal, which puddle has a strong tendency to assume the equilibrium shape of a droplet on account of the high surface tension of molten metal; they are prone to variations in width because even slight unavoidable variations in the flow rate of the molten metal through the orifice to form the jet will cause variations in the diameter of the puddle with resultant variations in width of the strip drawn therefrom.
U.S. Pat. No. 3,862,658 to Bedell inter alia discloses a method for making amorphous strips (filaments) by ejecting molten alloy into the nip of two closely spaced counter-rotating steel rolls. This method provides for rapid, effective cooling of the melt, but involves rolling of the solidified strip between the steel rolls, as a result of which the product has anisotropic tensile properties. By that method Bedell obtained an amorphous ribbon of 0.012 centimeters thickness and 1.27 centimeters width (Example 4 of U.S. Pat. No. 3,862,658).
British Patent No. 20,518 to Strange, and U.S. Pat. No. 905,758 to Strange and Pim further illustrate processes for making sheets, foils, strips, or ribbons of metals by depositing molten metal onto a moving chill surface.